(a) Technical Field
The present invention relates to a lead-free solder composition for glass. More particularly, it relates to an eco-friendly lead-free solder composition for glass that lacks heavy metals such as lead, while preventing glass cracking during vehicle operation.
(b) Background Art
Generally, glass is used on all sides of a vehicle to ensure a driver's view of the road and surrounding environment. In particular, the glass used in the front (e.g., windshield) and the back (e.g., rear window) of the vehicle is typically subject to additional vehicle functionality such as, for example, a defroster, radio antenna, etc. associated with the windshield and a heating element, antenna, etc. associated with the rear window.
In order to provide these types of functionalities to vehicle's windows, it is necessary to provide electrical connections for power supply and electrical signal transmission. In order to stably supply electricity to a vehicle's glass window, a terminal for supplying power from the battery to an electrode formed on/in the glass surface must be soldered. Unfortunately, the terminal and the glass are formed of different materials that have significantly different thermal expansion coefficients. Under vehicle operating temperatures that may range from about −40° C. to about +105° C., the difference in the thermal expansion rates of these materials may result in the glass cracking.
Accordingly, lead (Pb), which is a soft metal, is typically added to a solder for glass in order to disperse the thermal stress that occurs between the glass and the terminal. For example, in the case of windshield glass, heating element wires are disposed in the lower part of glass to prevent the wiper blades from freezing in winter. In the case of side window glass, an antenna may be disposed on the side window glass of vehicles such as panel vans. Similarly, in the case of rear window glass, heat wires, antenna, etc. may disposed.
The heating element wire and the antenna are usually formed by printing a paste including silver (Ag) as a main ingredient, and sintering the paste in glass. The heating element wire and the antenna are soldered with terminals for an electric connection. At present, a leaded solder, i.e., a solder including a large quantity of lead (Pb), such as tin (Sn)-62 lead (Pb)-3 silver (Ag)-10 bismuth (Bi) composition is conventionally used.
Recently, SOx and NOx are being emitted in great quantities into the atmosphere due to increase of the use of fossil fuels such as gasoline or heavy oil. These airborne pollutants have the effect of lowering the pH of rain, causing acid rain. Unfortunately, when acid rain comes into contact with the soldered parts of electronic equipments on the ground, it can cause the lead to melt out of a lead-tin alloy solder and penetrate into the ground, thereby contaminating the groundwater with heavy metals (e.g., Pb). Such heavy metal contamination may lead to a variety of health conditions in people who consume the contaminated water, such as lead poisoning. Accordingly, a lead-free solder is needed.
In the case of lead-free solder, a three (Sn—Ag—Cu) element-based solder such as tin (Sn)-3.0 silver (Ag)-0.5 copper (Cu) composition is generally used. Unfortunately, this composition has a limitation in glass-metal applications because cracks occur in the glass as a result of a stress generated when the lead-free solder melted at a high temperature is hardened in a glass substrate during the soldering process. This is caused by a difference of the expansion coefficient between glass and an alloy including tin (Sn) and other metals.
Also, U.S. Pat. No. 6,253,988 discloses a 4-element-based lead-free solder composition of tin (Sn)-silver (Ag)-copper (Cu)-indium (In) composition for use in vehicle glass applications, which provides a composition of tin (Sn)-4.5 silver (Ag)-0.5 copper (Cu)-65 indium (In). Unfortunately, this composition is of limited commercial value because contains significant levels of indium (In) and silver (Ag), and is therefore cost prohibitive.